1. Field of the Invention.
This invention relates in general to magnetic sensors, and more particularly to a method and apparatus for providing a current-in-plane (CIP) GMR sensor with an improved in-stack bias layer with a thinner sensor stack.
2. Description of Related Art.
Magnetic recording is a key segment of the information-processing industry. While the basic principles are one hundred years old for early tape devices, and over forty years old for magnetic hard disk drives, an influx of technical innovations continues to extend the storage capacity and performance of magnetic recording products. For hard disk drives, the areal density or density of written data bits on the magnetic medium has increased by a factor of more than two million since the first disk drive was used for data storage. Areal density continues to grow due to improvements in magnetic recording heads, media, drive electronics, and mechanics.
Magnetic recording heads have been considered the most significant factor in areal-density growth. The ability of the magnetic recording heads to both write and subsequently read magnetically recorded data from the medium at data densities well into the gigabits per square inch (Gbits/in2) range gives hard disk drives the power to remain the dominant storage device for many years to come.
Important components of computing platforms are mass storage devices including magnetic disk and magnetic tape drives, where magnetic tape drives are popular, for example, in data backup applications. Write and read heads are employed for writing magnetic data to and reading magnetic data from the recording medium. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
A magnetoresistive (MR) sensor changes resistance in the presence of a magnetic field. Recorded data can be read from a recorded magnetic medium, such as a magnetic disk, because the magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the read element, which causes a corresponding change in the sensor resistance.
A magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a sensing element as a function of the strength and direction of magnetic flux being sensed by the sensing element. Conventional MR sensors, such as those used as MR read heads for reading data in magnetic recording disk and tape drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy. A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the magnetic disk in a magnetic disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element. This change in resistance may be used to detect magnetic transitions recorded on the recording media.
In the past several years, prospects of increased storage capacity have been made possible by the discovery and development of sensors based on the giant magnetoresistance (GMR) effect, also known as the spin valve effect. In a spin valve sensor, the GMR effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free-layer. Magnetic sensors utilizing the GMR effect are found in mass storage devices such as, for example, magnetic disk and tape drives and are frequently referred to as spin valve sensors. In operation, a sense current is caused to flow through the read head and therefore through the sensor. The magnetic flux from the disc causes a rotation of the magnetization vector in at least one of the sheets, which in turn causes a change in the overall resistance of the sensor. As the resistance of the sensor changes, the voltage across the sensor changes, thereby producing an output voltage.
The output voltage produced by the sensor is affected by various characteristics of the sensor. The sense current can flow through the sensor in a direction that is parallel to the planes of the layers or stacked strips. This is known as a current-in-plane (CIP) configuration.
Alternatively, the sense current can flow through the sensor in a direction that is perpendicular to the planes of the layers or stacked strips that comprise the sensor. This configuration is known as a current-perpendicular-to-plane (CPP) configuration.
One of the problems with such a MR read head, however, lies in developing a structure that generates an output signal that is both stable and linear with the magnetic field strength from the recorded medium. If some means is not used to stabilize the sensing ferromagnetic layer, i.e., to maintain it in a single magnetic domain state, the domain walls of magnetic domains will shift positions within the sensing ferromagnetic layer, causing noise that reduces the signal-to-noise ratio. This may give rise to a non-reproducible response of the head, when a linear response is required. Therefore, the free-layer must be stabilized by longitudinal biasing so that the magnetic spins of the free-layer are in a single domain configuration.
There are two stabilization schemes for biasing of the free-layer. One stabilization scheme is to provide a bias field from the lead regions at the side edges of the read sensor. The most common technique includes the fabrication of a bias layer for providing tail stabilization at the physical track edges of the sensor. The efficacy of the method of stabilization depends critically on the precise details of the tail stabilization, which is difficult to accurately control.
The other stabilization scheme is to provide an in-stack longitudinal bias structure including a soft ferromagnetic bias layer and an anti-ferromagnetic (AFM) bias layer. However, as the thickness of the AFM layer is reduced, the exchange bias also decreases. One method to construct thinner sensors is to provide biasing without an anti-ferromagnetic layer using in-stack biasing. However, the sense current in a current-in-plane (CIP) GMR sensor may be shunted by the bias stack thereby degrading the ability of the sensor to detect recorded signals.
It can be seen that there is a need for a method and apparatus for providing a current-in-plane (CIP) GMR sensor with an improved in-stack bias layer with a thinner sensor stack.